1. Field of the Invention
The present invention relates to an optical disc which can be manufactured under uniform conditions by forming grooves and lands on the entire surface of the disc having a lead-in area, a user data area and a lead-out area, and which is configured to obtain a highly reliable recording/reproduced signal.
2. Description of the Related Art
In general, optical discs are widely employed as information recording media for an optical pickup device which records/reproduces information in a non-contact manner. They are classified into compact discs (CDs) and digital versatile discs (DVDs) according to information recording capacity. Furthermore, a DVD disc capable of writing, erasing and reading information can be sub-divided into a digital versatile disc-random access memory (DVD-RAM) disc and a digital versatile disc-rewritable (DVD-RW) disc.
FIG. 1 shows a conventional DVD-RAM or DVD-RW disc having a lead-in area 10, a user data area 20 and a lead-out area 30. The lead-in area 10 contains read only data, such as the disc size, number of track layers on a readable plane or illegal copy preventing information. The user data area 20 contains user data that can be repeatedly read and/or written. The lead-out area 30 contains other disc-related information.
FIG. 1 further shows a partially enlarged view of the lead-in area 10 (a portion A), the user data area 20 (a portion C) and the lead-out area 30 (a portion B). In the lead-in area 10 and the lead-out area 30, pits 15 are used to record read only data.
In the user data area 20, grooves 23 and lands 25 are alternatively formed to accommodate recording and/or reproducing information marks 27 along a predetermined track. Here, a reference numeral 40 denotes a reproduction beam.
A noticeable difference between a DVD-RAM and a DVD-RW is a physical area provided for recording. In other words, the DVD-RAM performs recording on both the lands 25 and the grooves 23, while the DVD-RW performs recording only on the grooves 23. Application of these two standard formats results in the following problems.
First, while a DVD-RW having the same physical recording structure as a DVD-ROM (read only disc) has an excellent reproduction compatibility in DVD-ROM drives or DVD players, a DVD-RAM having a phase difference corresponding to depths of a land and a groove requires hardware modification to suitably track lands and grooves. Therefore, a conventional DVD-RAM has a poor reproduction compatibility.
Second, in the context of recording/reproduction characteristics or injection-molding characteristics in recording data on a groove, the grooves formed in a DVD-RW are two or more times shallower than that in a DVD-RAM. Here, if necessary, read only data is formed on the lead-in area 10 in a form of pits 15.
FIG. 2 shows a graph illustrating an amplitude ratio of a reproduced signal with respect to a pit depth represented in λ/n unit for a wavelength (λ) of a reproduced beam to a refractive index (n) of a disc. In cases where the lengths of a recording mark for the minimum recording mark length T are 3T and 14T, the amplitude ratios denoted by m1 and m2 are in a range of between 0.2 and 0.3 where the pit depth (corresponding to a groove depth of a DVD-RW) is approximately 0.06 in λ/n unit. The amplitude ratio is approximately 1 where the pit depth is approximately 0.25. Accordingly, the signal level at the pit depth of λ/12n is approximately 30% (1:0.3) as compared to the case where the pit depth is λ/4n. Therefore, a reliable pit signal cannot be obtained where read only data as shallow as a groove depth of a DVD-RW is formed in a DVD-RAM.
Third, there is a demand for a multi-layered optical disc having a plurality of recording layers, looking from the direction of an incident beam, to enhance the recording capacity. FIG. 3 shows a dual recording layer disc having a first recording layer L0 and a second recording layer L1. A recording laser passes through the first recording layer L0 where a recording is performed on the second recording layer L1. In this case, there is a difference in light power between a pit portion and a groove portion. Also, where a physical header representing a basic recording unit in a data area is used, there is a difference in light transmittance because unlike the recording area, the physical header area always remains crystallized.
FIG. 4 shows a graph illustrating light power for each of a mirror portion, pit portion, groove portion and a groove portion with marks. As shown in FIG. 4, the physical geometry of the first recording layer L0 affects the light power.
Table 1 below lists conditions used in the light power experiments.
TABLE 1ParameterConditionWavelength (nm)400Numerical aperture (NA)0.65/0.85Minimum mark length (μm)0.275/0.194ModulationEFM+ (Eight-to-Fourteen Modulation-plus)Track pitch (μm)0.30, 0.34, 0.38Reflectivity (%)Rc = 25, Ra = 5
In Table 1, Rc represents the reflectivity of a crystallized portion of a recording layer and Ra represents the reflectivity of an amorphous portion of a recording layer. According to the experimental results, the smallest decrease in the light power was found in the mirror portion. The light power gradually decreased, in order, with the physical geometry of a pit portion, a groove portion and a groove portion with marks. FIG. 3 shows that a recording/reproducing beam 40 is trapped over a boundary of the lead-in area 10 of the first recording layer L0 and the data area 20 having grooves. Accordingly, the amount of the light beam irradiated onto the second recording layer L1 is different from the case where a recording/reproducing beam 40 extends over only to the grooves. Therefore, the groove portion with marks adversely affects the recording power as the data is written on the second recording layer L1 of the dual-layered optical disc, resulting in a poor recording/reproduction efficiency.
Fourth, in order to reduce a spot size of a reproducing beam to attain high-density, a numerical aperture (NA) should be increased. However, the problem with a dual recording layer disc is that a difference in light power becomes more serious as the NA increases. Factors causing the difference in the light power with increased NA are listed in Table 2 below.
TABLE 2ItemParameterExampleDual recordingStructure of first recording layerGrooves, pits, etc.,layersHigh NANumber of tracks trapped by beam  85 for NA 0.65 160 for NA 0.85Incident angle of beam40.5° for NA 0.6558.2° for NA 0.85
As shown in Table 2, with the grooves and pits formed on the first recording layer of a dual recording layer disc, the number of tracks trapped by a beam and the incident beam angle increase as the NA is increased.
Finally, the manufacturing conditions of the disc mastering may vary depending on different structures of the disc in a lead-in area (pits), a data area (grooves) and a lead-out area (pits). This makes the manufacturing process complex, resulting in a poor yield and an increased manufacturing cost.